dict init/update accesses internal items of dict derivative

[blakeross]

BPO	1730480
Nosy	@rhettinger, @terryjreedy, @devdanzin

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2010-08-09.04:02:30.774>
created_at = <Date 2007-06-04.03:59:53.000>
labels = ['interpreter-core', 'type-feature']
title = 'dict init/update accesses internal items of dict derivative'
updated_at = <Date 2010-08-11.20:24:29.695>
user = 'https://bugs.python.org/blakeross'

bugs.python.org fields:

activity = <Date 2010-08-11.20:24:29.695>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = <Date 2010-08-09.04:02:30.774>
closer = 'terry.reedy'
components = ['Interpreter Core']
creation = <Date 2007-06-04.03:59:53.000>
creator = 'blakeross'
dependencies = []
files = []
hgrepos = []
issue_num = 1730480
keywords = []
message_count = 4.0
messages = ['32207', '85623', '113372', '113627']
nosy_count = 4.0
nosy_names = ['rhettinger', 'terry.reedy', 'blakeross', 'ajaksu2']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'test needed'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue1730480'
versions = ['Python 3.1', 'Python 2.7']

[blakeross]

>>> class MyDict(dict):
...     def keys(self): print "keys"
...     def __getitem__(self, n): print "__getitem__"
...
>>> myDict = MyDict(a=1, b=2)
>>> dict(myDict)
{'a': 1, 'b': 2}

PyDict_Merge accesses the items of the dict to be merged directly rather than going through the interface for any dict instance--even a dict derivative--by virtue of using PyDict_Check rather than PyDict_CheckExact. I believe the logic needs to be:

if type(d).__getitem__ is dict.__getitem__ and type(d).keys is dict.keys:
    ...okay to access items directly...
else:
    ...go through the methods...

[devdanzin]

IIRC, the same behavior is present for subclasses of other builtin types.

[terryjreedy]

It is documented that access to special methods is more direct than normal path. This is not going to change. A similar example was discussed on python-list earlier this year with the above conclusion.

[rhettinger]

I concur with Terry.
This is just a fact of life when subclassing builtins.
The objects are "open-for-extension, closed-for-modification".
If you want more direct control use UserDict.